Various devices of this general type have heretofore been provided, however, because of certain inherent design characteristics they have been beset with one or more of the following shortcomings: (a) they are of a costly and complex design; (b) they are highly susceptible to malfunction and require an inordinate amount of maintenance and service; (c) they are difficult to install and require substantial modifying of the existing structure on which the device is to be used; and (d) electric motors, hydraulic pumps, and various types of control switches are required to operate the device.